We have developed a method for short-term culture of Pneumocystis carinii using the CCL 64 mink lung cell line as a feeder cell layer. This culture method has enabled us to obtain a relatively pure Pneumocystic carinii trophozoite population in culture. This population has been surface labeled using a 125I Iodogen method. A 120 Kilodalton glycoprotein (p120) has been identified and purified. It is a major surface protein. We propose to obtain the primary structure of this protein. We will use this information to synthesize a DNA consensus probe. This probe will be used to identify and study the structure of the p120 gene and will also be used in a hybridization assay for P. carinii. We have produced antibodies in rabbits to the p120 protein and propose to develop a quantitative ELISA assay to measure the p120 protein in pulmonary lavage fluid. Pneumocystis pneumonia is the commonest fatal complication of the Acquired Immunodeficiency Syndrome AIDS. There are at present no quantitative assays for P. carinii.